Kinetic analysis of adrenal tyrosine hydroxylase (TH) with respect to varying concentration of 6-methyltetrahydropterin cofactor demonstrated two distinct forms of the enzyme-one with a relatively low cofactor affinity and a high V max and another with a 100-fold greater cofactor affinity and a low V max. Intial Scatchard analysis by computer suggests that phosphorylation of adrenal TH produces qualitatively distinct changes in the kinetics of these 2 forms of TH. Passing a crude adrenal homogenate over Dowex-50 seems to convert the enzyme to single kinetic form with intermediate Km's and V max's.